It is known that considerable efforts have been expended to reduce corrosion in metals. Thus, the use of triazinecarboxylic acids has been proposed in European Patent 46 139 as corrosion inhibitors for aqueous systems in contact with iron or iron-containing metals. European Patent 511 163 discloses fluid aqueous dispersions containing a solid polycarboxyltriazine acid corrosion inhibitor. Furthermore, U.S. Pat. No. 4,402,907 and European Patent A-129 506 disclose the suitability of certain heterocyclic polycarboxylic acids as corrosion inhibitors for aqueous systems in contact with metals, which can be used in aqueous systems, for example, in cooling water systems, steam generating systems, metalworking means, and aqueous hydraulic fluids. Since most polycarboxylic acids are only slightly soluble in water, the polycarboxylic acids are used in the form of their water-soluble salts, i.e., they are neutralized before use or added to a basic aqueous system. They are, however, usually stored and sold in the form of free polycarboxylic acids.
Free polycarboxylic acids are usually solid substances. They are normally isolated from an aqueous phase by filtration during manufacturing. The filtered product is generally rinsed with water and then dried. More recently, in order to save energy needed for drying, the wet filter cake, containing approximately 50% water, has been offered commercially for use in aqueous systems. The wet filter cake, however, has the disadvantage that it is not fluid. It cannot be poured or cast, but must be metered or refilled manually with the help of a scoop. Therefore it has also been suggested that polycarboxylic acids be used in the form of high-solids aqueous dispersions.
Melamine-based corrosion inhibitors are also known. Thus, U.S. Pat. No. 2,485,309 describes methylolmelamine condensation products, but these by no means meet today's strict requirements for a corrosion inhibitor.
Furthermore, Chemical Abstracts 126:52 599 and 124:98 444 disclose reaction products of one mole melamine with one or two moles of succinic acid. Their use as corrosion protection agents, however, is not mentioned.